The present invention relates to a linear accelerator.
Linear accelerators, particularly of the standing wave design, are known as a source of an electron beam, for example for use in X-Ray generation. This beam can be directed to an X-ray target which then produces suitable radiation. A common use for such X-rays or for the electron beam is in the medical treatment of cancers etc.
It is often necessary to vary the incident energy of the electron beam on the X-ray target. This is particularly the case in medical applications where a particular energy may be called for by the treatment profile. Linear standing wave accelerators comprise a series of accelerating cavities which are coupled by way of coupling cavities which communicate with an adjacent pair of accelerating cavities. According to U.S. Pat. No. 4,382,208, the energy of the electron beam is varied by adjusting the extent of rf coupling between adjacent accelerating cavities. This is normally achieved by varying the geometrical shape of the coupling cavity.
This variation of the geometrical shape is typically by use of sliding elements which can be inserted into the coupling cavity in one or more positions, thereby changing the internal shape of the cavity. There are a number of serious difficulties with this approach arising from the various other resonant parameters that are dictated by the cavity dimensions. Often more than one such element has to be moved in order to preserve the phase shift between cavities at a precisely defined value. The movement of the elements is not usually identical, so they have to be moved independently, yet be positioned relative to each other and the cavity to very great accuracy in order that the desired phase relationship is maintained. Accuracies of xc2x10.2 mm are usually required. This demands a complex and high-precision positioning system which is difficult to engineer in practice. In those schemes which have less than two moving parts (such as that proposed in U.S. Pat. No. 4,286,192), the device fails to maintain a constant phase between input and output, making such a device unable to vary RF fields continuously, and are thus reduced to the functionality of a simple switch. They are in fact often referred to as an energy switch.
Many of these schemes also propose sliding contacts which must carry large amplitude RF currents. Such contacts are prone to failure by weld induced seizure, and the sliding surfaces are detrimental to the quality of an ultra high vacuum system. Issues of this nature are key to making a device which can operate reliably over a long lifetime.
The nature of previous proposed solutions can be summarised as cavity coupling devices with one input and one output hole, the whole assembly acting electrically like a transformer. To achieve variable coupling values the shape of the cavity has had to be changed in some way, by means of devices such as bellows, chokes and plungers. However the prior art does not offer any device which can vary the magnitude of the coupling continuously over a wide range by means of a single axis control, while simultaneously maintaining the phase at a constant value.
The present state of the art is therefore that such designs are accepted as providing a useful way of switching between two predetermined energies. However, it is very difficult to obtain a reliable accelerator using such designs that offers a truly variable energy output.
A good summary of the prior art can be found in U.S. Pat. No. 4,746,839.
The present invention therefore provides a standing wave linear accelerator, comprising a plurality of resonant cavities located along a particle beam axis, at least one pair of resonant cavities being electromagnetically coupled via a coupling cavity, the coupling cavity being substantially rotationally symmetric about its axis, but including a non-rotationally symmetric element adapted to break that symmetry, the element being rotatable within the coupling cavity, that rotation being substantially parallel to the axis of symmetry of the coupling cavity.
In such an apparatus, a resonance can be set up in the coupling cavity which is of a transverse nature to that within the accelerating cavities. It is normal to employ a TM mode of resonance with the accelerating cavities, meaning that a TE mode, such as TE111, can be set up in the coupling cavity. Because the cavity is substantially rotationally symmetric, the orientation of that field is not determined by the cavity. It is instead fixed by the rotational element. Communication between the coupling cavity and the two accelerating cavities can then be at two points within the surface of the coupling cavity, which will xe2x80x9cseexe2x80x9d a different magnetic field depending on the orientation of the TE standing wave. Thus, the extent of coupling is varied by the simple expedient of rotating the rotational element.
Rotating an element within a vacuum cavity is a well known art and many methods exist to do so. This will not therefore present a serious engineering difficulty. Furthermore, eddy currents will be confined to the rotational element itself and will not generally need to bridge the element and its surrounding structure. Welds will not therefore present a difficulty.
The design is also resilient to engineering tolerances. Preliminary tests show that an accuracy of only 2 dB is needed in order to obtain a phase stability of 2% over a 40xc2x0 coupling range. Such a rotational accuracy is not difficult to obtain.
It is preferred if the rotational element is freely rotatable within a coupling cavity of unlimited rotational symmetry. This arrangement gives an apparatus which offers greatest flexibility.
A suitable rotational element is a paddle disposed along the axis of symmetry. It should preferably be between a half and three quarters of the cavity width, and is suitably approximately two-thirds of the cavity width. Within these limits, edge interactions between the paddle and the cavity surfaces are minimised.
The axis of the resonant cavity is preferably transverse to the particle beam axis. This simplifies the rf interaction considerably.
The accelerating cavities preferably communicate via ports set on a surface of the coupling cavity. It is particularly preferred if the ports lie on radii separated by between 40xc2x0 and 140xc2x0. A more preferred range is between 60xc2x0 and 120xc2x0. A particularly preferred range is between 80 and 100xc2x0, i.e. approximately 90xc2x0.
The ports can lie on an end face of the cavity, i.e. one transverse to the axis of symmetry, or on a cylindrical face thereof. The latter is likely to give a more compact arrangement, and may offer greater coupling.
Thus, the invention proposes the novel approach of coupling adjacent cells via a special cavity operating in a TE mode, particularly the TE111 mode. By choosing the coupling positions of the input and output holes to lie along a chord of the circle forming one of the end walls of the cavity, a special feature of the TE111 mode can be exploited to realise a coupling device with unique advantages. Instead of changing the shape of the cavity, this invention proposes to rotate the polarisation of TE111 mode inside the cavity by means of a simple paddle. Because the frequency of the TE111 mode does not depend upon the angle that the field pattern makes with respect to the cavity (the polarising angle), the relative phase of RF coupled into two points is invariant with respect to this rotation, at least over 180xc2x0. At the same time, the relative magnitude of the RF magnetic fields at the two coupling holes lying along a chord varies by up to two orders of magnitude. This property of the RF magnetic field is the basis of the variable RF coupler of this invention.
The key to the proposed device is that the moving paddle is not a device to change the shape of the cavity, as described in the prior art, but is merely a device to break circular symmetry of the cylindrical cavity. As such the paddle does not have to make contact with the walls of the cavity, nor does any net RF current flow between the paddle and the cavity wall. This makes the device simple to construct in vacuum, requiring only a rotating feed-through, which is well known technology. Alternatively, the paddle might be rotated by an external magnetic field, and so eliminate the vacuum feed-through requirements entirely.